The growth kinetics of preexposed and naive Tetrahymena pyriformis grown in the presence of one hydrophilic and one hydrophobic nonpolar narcotic (acetone and 2-decanone, respectively) have been evaluated. The response of naive Tetrahymena exposed to nonpolar narcotics varied from a change in generation time upon exposure to hydrophilic chemical to a change in lag phase with similar generation time compared to control upon exposure to hydrophobic compounds. Tetrahymena grown in the presence of low concentrations of 2-decanone and then transferred to higher concentrations acclimated to the presence of the toxicant. Acclimation was demonstrated by reduced lag phases compared to naive cells. Results of population growth studies of Tetrahymena grown in the presence of low concentrations of acetone and then transferred to higher concentrations of acetone exhibit the same pattern, an increased generation time with increasing concentration with no lag time, as naive populations. Additionally, the observed generation times in acetone were cumulative relative to the transfer concentration as well as the acclimation concentration. The most feasible explanation for this phenomenon is the interaction of the toxicants with the plasma membrane.